1. Field of the Invention
This invention relates to electrically conductive materials and particularly to a conductive rubber and more particularly to an extension type conductive rubber that decreases its electrical resistance when extended.
2. Prior Art
Heretofore, numerous pressure sensitive conductive rubbers have been developed that contain suspended carbon or conductive metal particles to decrease electrical resistance when compressed. Most of these pressure sensitive conductive rubbers, however, have been of a switching type, that exhibit a sharp decrease in electrical resistance when a pressure is applied thereto. Which material can accordingly present only two discrete states, a high resistance state and a low resistance state. Applications of such switching type conductive rubber are limited to switching devices that perform an on-off action. They cannot be used as a pressure sensor that is required to sense intermediate values. Switching type conductive rubber exhibits such characteristics because it contains larger metal particles having diameters of 30 to 200 microns and the interparticle distances between which particles are small. In addition, the switching type conductive rubber cannot be stretched as it is easily ruptured when stretched due to the larger conducting particles contained therein. Switching type conductive rubber therefore will not present a usable change in its electrical resistance by stretching, as does the extension type rubber of the present invention.
Japanese Patent Publication No. Sho 60-33138, discloses a pressure sensitive conductive rubber formed by impregnating a vulcanized conductive rubber, such as a butadiene rubber (BR) or nitrile rubber (NBR) containing a conductive filler, for example, carbon black or fine metal powders, with a nonvolatile oil. Which nonvolatile oil has compatibility with the conductive rubber so as to swell the rubber and to enlarge interparticle distances of the conductive filler therein. This pressure sensitive conductive rubber, unlike the above-mentioned switching type conductive rubber, gradually decreases its electrical resistance when a pressure applied thereto is increased. Also, it can present a high, intermediate and low electrical resistance states in response to pressure. However, it has shortcomings, that will be discussed later herewith when used as a pressure sensor.
In use, a pressure sensitive conductive rubber is sandwiched between a pair of opposing electrodes. Thereby, when a compression force is applied to the electrodes, the electrical resistance across the electrodes will decrease in response to the magnitude of the applied force. In such arrangement, the electrical resistance across the electrodes involves a contact resistance between the electrodes and the rubber as well as the electrical resistance of the rubber itself. Accordingly, as the compression force is increased, there occurs a slip between surfaces of the electrodes into closer contact with the rubber, thus decreasing the contact resistance. A slip also occurs therebetween as the compression force is decreased, that in turn lessens the contact of the electrodes with the rubber, thus increasing the contact resistance therebetween. In practice, when the compression force is increased, a mechanical frictional resistance between the electrodes and rubber delays the slip and the resultant decrease of the contact resistance, thereby also delaying the decrease of the electrical resistance across the electrodes. Similarly, when the compression force is decreased, the frictional resistance delays the slip and the resulting increase in contact resistance, thereby also delaying the increase of the electrical resistance across the electrodes. Thus, the relation between the force and the electrical resistance across the electrodes presents a hysteresis. Thereby a value of the electrical resistance across the electrodes, i.e., the apparent electrical resistance of the rubber may vary for the same magnitude of applied force. Moreover, a hysteresis loss of the rubber itself due to the creep and stress relaxation is superposed on the above-mentioned hysteresis. Therefore, this pressure sensitive conductive rubber cannot be reliably utilized for a sensor requiring high degree of accuracy.
In addition, the pressure sensitive conductive rubber exhibits poor elasticity and is easily ruptured when stretched. Accordingly, it is not reliable when stretched, as is the extension type conductive rubber of the present invention.